GB2262945A

GB2262945A - Thermal spraying of piston rings with molybdenum alloy

Info

Publication number: GB2262945A
Application number: GB9226854A
Authority: GB
Inventors: Jan Vatavuk; Jose Roberto Zalli Neto; Marcos R Piccili
Original assignee: COFAP Companhia Fabricadora de Pecas Ltda
Current assignee: Marelli Cofap do Brasil Ltda
Priority date: 1992-01-03
Filing date: 1992-12-23
Publication date: 1993-07-07
Also published as: BR9200089A; JPH0693409A; NL9202300A; ES2053397B1; ES2053397A1; FR2685923B1; PT101164A; GB9226854D0; MX9207674A; DE4244610A1; FR2685923A1

Abstract

Thermal spraying lining process for piston rings, including the initial step of anchoring a plurality of piston heads (1) on a basic structure (2), against mutual relative displacements, so that their external contact faces (1a) define a cylindrical surface, and further including the step of spraying, progressively and continuously, by a forced gaseous stream and onto the cylindrical surface of the basic structure (2), a mass of droplets which are melted by an oxy-acetylene torch, from at least one molybdenum feeding wire (4), and from at least one feeding wire (4) of a softer metallurgical bonding material which may be steel, the mass of droplets consisting of about 30 to 60% of molybdenum, the remaining being bonding material. The cylindrical surface may be subjected, during spraying, to a rotational motion around its geometrical axis, simultaneously with a translational motion towards said geometrical axis, in order to produce helical paths to the mass of sprayed droplets. <IMAGE>

Description

THERMAL SPRAYING LINING PROCESS FOR PISTON RINGS Technical Field The present invention relates to a thermal spraying lining process for piston rings, which uses different materials, presents an operational low cost and maintains unalterable the average lining properties.

Background Art A known thermal spraying lining process for piston rings employs 99.9% pure molybdenum feeding wires, which are melted by respective torches of oxygen and acetylene. In this process, droplets of melted molybdenum are sprayed against the external contact faces of piston rings, said faces presenting a relatively low temperature as compared to that of the droplets upon reaching said external contact face, compressed air being used as propeller. The adhesion between the droplets and the external contact face is mechanically achieved, by the solidification of said droplets onto the rugosity of said external contact face, due to the high contraction tension during solidification.

Besides the high cost of this process, which uses only molybdenum, the resultant lining layer proves to be fragile, due to the fact that the welding of successive lining layers occurs through oxide-rich interfaces. Such fragility makes the lining susceptible to cracks and chips, when the piston ring is submitted to mechanical stresses. The integrity of the lining with molybdenum is affected by the operational high temperatures of said piston ring close to the combustion chamber, mainly due to the substantial difference between the thermal expansion coefficient of the molybdenum and the basic metal of iron alloy, giving rise to said cracks when under stress.

Moreover, the spraying lining process using wires is little versatile in relation to the combination of different materials.

Another known process employs powder spraying, which enables the association of materials, but has a very high cost, mainly in the case of spraying by means of plasm arc.

Summary of the Invention Thus, it is an object of the present invention to provide a thermal spraying lining process for piston rings, which produces a lining that is resistant to cracks and chips when submitted to large mechanical stresses, and of low cost.

Another object of the present invention is to provide a thermal spraying lining process for piston rings, presenting more versatility in relation to the combination of different materials, thereby enabling the composition of different properties.

These objects are achieved through a thermal spraying lining process for piston rings, which comprises the initial step of anchoring a plurality of piston rings on a basic structure against mutual relative displacements, so that their external contact faces define a cylindrical surface, and a complementary step of spraying, progressively and continuously, by a forced gaseous stream, and onto the entire extension of said cylindrical surface, a mass of droplets, which are melted by an oxyacetylene torch, from at least one molybdenum feeding wire, and from at least one feeding wire of a softer metallurgical bonding material, the mass of droplets consisting of about 30 to 60% of molybdenum, the remaining being bonding material, said spraying being made, so as to integrally line the cylindrical surface with the mass of droplets in at least one layer.

Brief Description of Drawings The invention will be now described, according to the attached drawings, in which: Fig. 1 shows a partial longitudinal section view of a piston operating inside a cylinder and provided with piston rings, the pressure of the gases being indicated by the arrow; and an enlarged detail of one of said piston rings, enhancing the lining of its external contact face; Fig. 2 illustrates, schematically, the piston rings being sprayed by means of feeding wires, according to the present invention; and Fig. 3 shows an enlarged cross sectional view of part of a piston ring provided with lining on its external contact face.

Disclosure of the Invention According to the present invention, there are provided piston rings 1, to be mounted on a reciprocating piston P and which operate, during combustion, close to the internal wall of a cylinder C. Each piston ring 1 presents an external contact face la provided with an anti-adhesive and anti-abrasive lining, which protects said piston rings 1 when they are submitted to high pressure and temperature during the combustion cycle.

In one embodiment of the invention, the external contact face la of each piston ring 1 is initially produced by machining said piston ring 1, in order to make a superficial annular groove at the circular mid portion of said piston ring 1, said groove being later filled with the lining, until the diameter of the external contact face la of the piston ring 1 is recuperated.

In another way of carrying out the invention, the lining layer is externally provided on the external contact face la, thus increasing the diameter of the part at the region where the lining is applied, until there is obtained a lining layer with an annular width sufficient to have, after final machining, a lining thickness from 2 to 7% of the basic annular width of the ring.

As illustrated in figure 2, a basic structure 2 carries and anchors, along its length, by using retaining means such as bolts and nuts, a plurality of piston rings 1, which are restrained from mutual relative displacements, and which are arranged so as to define a cylindrical surface presenting a rotational motion around a rotation shaft, which passes through the center of the ring set and which is coincident with the geometrical axis of the basic structure 2, and a translational motion, towards said geometrical axis.

At a certain distance from said basic structure 2, there are provided two gas siphons 3, which are simultaneously fed through their respective inlet a, b, with oxygen and acetyleno, so as to form a melting flame capable of melting the chemical elements used in the present spraying process. Each gas siphon 3 presents a flame regulation, which is achieved by varying the proportions of oxygen and acetylene in the flame.

Besides the gases which are fed to the inside of siphons 3, in order to form the flame described above, each siphon 3 receives, through third and fourth inlets c and d, respectively, chemical- elements to be melted and used in the lining of said piston rings 1, and a volume of compressed air which carries, through an outlet "s" of each said siphon 3, the droplets of each melted chemical element, towards the external contact face of the cylindrical surface of the basic structure 2.

In a preferred way of carrying out the invention, the lining is obtained through the simultaneous thermal spraying of two chemical elements, which are different but have the same concentration, and which are presented in the fdrm of feeding wire rolls 4, respectively passing through a siphon 3.

In the preferred described solution, one of the chemical elements used in the lining is molybdenum, which is responsible for the anti-abrasive and anti-adhesive characteristics of the lining, and which is employed in a condition of 99.9% purity. The other of said materials is stainless steel. This element, which presents a lower hardness in relation to molybdenum, is responsible for the metallurgical bonding of molybdenum to the external contact face la of the piston rings 1, and for the cohesion of several lining layers applied over anterior layers. For each feeding wire that is used, the oxyacetylene flame has a specific regulation, obtained through an adequate proportion of oxygen and acetylene defined according to the melting point of each element for the composition of the feeding wires.

The stainless steel, which presents a lower melting point, makes possible a better metallurgical bonding, increasing the strength against cracks in the lining under mechanical stresses, improving the adhesion thereof to the basic metal of the piston ring 1, as well as the cohesion of the sprayed particles.

The adhesion of the lining to the basic metal is mainly metallurgical. In this adhesion, the molybdenum droplets, when sprayed onto the external contact face of the piston rings, reach this surface, which is at a lower temperature, thus being solidified in an anchored condition to the rugosity to which the basic metal is previously submitted, due to its solidification.

According to the present invention, the lining is obtained by employing an alloy, in this case being a chemical metallurgical alloy of molybdenum and stainless steel, in order to obtain a better adhesion of molybdenum to the external contact face la of the piston rings, thus guaranteeing its resistance against cracks and breakages during the high working temperatures to which the piston rings 1 are subjected close to the combustion chamber.

The spraying takes place without interruption during a determined number of cycles of motions of the basic structure 2, until the external contact face la of each piston ring 1 of said basic structure 2 presents a homogeneous and continuous lining layer of the molybdenum/stainless steel mixture with an annular width, as previously described.

In the present solution, the lining is obtained by providing the simultaneous spraying of molybdenum and stainless steel, taking into account the points that are longitudinally displaced throughout the extension of the basic structure 2, so as to define, on the cylindrical surface of said basic structure 2, helical paths caused by the rotational and translational motions of said basic structure 2 in relation to the gas siphons 3 and, consequently, to the mass of droplets that are melted by spraying.

The stainless steel, not only guarantees the adhesion of molybdenum to the basic metal of the external contact face la of each piston ring 1, but also allows the bonding of the molybdenum droplets to each other.

Each feeding wire roll 4 has its own feeding speed for its respective siphon 3, defined according to the concentration required for the mixture of each chemical element used in the lining.

Moreover, the rotational speed of the basic structure 2 keeps a proportionality with the rotational speed of the feeding wire rolls 4 feeding the gas siphons 3, which is defined according to the number of layers and annular width of each lining layer to be metallurgically adhered to the external contact face la of each piston ring 1.

The modulus of said rotational speed of the basic structure 2, in relation to the modulus of the rotational speed of each feeding wire roll can be, for example, null.

After finishing the spraying of the molybdenum/stainless steel alloy onto the cylindrical surface, each piston ring 1 is submitted to machining at its external contact face la, in order to finish its working profile.

Claims

CLAIMS:

1. Thermal spraying lining process for piston rings, including the initial step of anchoring a plurality of piston heads (1) on a basic structure (2), against mutual relative displacements, so that their external contact faces (la) define a cylindrical surface, and further including the step of spraying, progressively and continuously, by a forced gaseous stream and onto the cylindrical surface of the basic structure (2), a mass of droplets which are melted by an oxy-acetylene torch, from at least one molybdenum feeding wire (4), and from at least one feeding wire (4) of a softer metallurgical bonding material, the mass of droplets consisting of about 30 to 60% of molybdenum, the remaining being bonding material.

2. Thermal spraying lining process for piston rings, including the initial step of anchoring a plurality of piston heads on a basic structure, against mutual relative displacements, so that their external contact faces define a cylindrical surface, it further includes the step of spraying, progressively and continuously, by a forced gaseous stream and onto the entire extension of said cylindrical surface of the basic structure, a mass of droplets which are melted by an oxyacetylene torch, from at least one molybdenum feeding wire, and from at least one feeding wire of a softer metallurgical bonding material, the mass of droplets consisting of about 30 to 60% of molybdenum, the remaining being bonding material, said spraying being made in order to integrally line said cylindrical surface with the mass of droplets in at least one layer, said cylindrical surface being subjected, daring spraying, to a rotational motion around its geometrical axis, simultaneously with a translational motion around said geometrical axis, in order to produce helical displacements of spraying points throughout the entire cylindrical surface.

3. Process, as in claim 1 or 2, wherein the metallurgical bonding material is stainless steel.

4. Process, as in claim 3, wherein the mass of droplets consists of substantially equal parts of molybdenum and stainless steel.

5. Process, as in claim 4, wherein the lining of the external contact face of said piston rings presents a final finished thickness, which corresponds from 2 to 7% of the annular width of the ring.

6. A piston ring made by a thermal spraying lining process substantially as hereinbefore described with reference to, and as illustrated in the accompanying drawings.

6. A Thermal spraying lining process substantially as hereinbefore described with reference to, and as illustrated, in the accompanying drawings.

7. A piston ring made by a thermal spraying lining process substantially as hereinbefore described with reference to, and as illustrated in the accompanying drawings.

Amendments to the claims have been filed as follows CLAIMS: 1. Thermal spraying lining process for piston rings, including the initial step of anchoring a plurality of piston heads on a basic structure, against mutual relative displacement, so that their external contact faces define a cylindrical surface, the process further including the steps of spraying, progressively and continuously, by a forced gaseous stream onto the entire extension of the cylindrical surface, a mass of droplets which are melted by an oxy-acetylene torch, from at least one molybdenum feeding wire, and from at least one feeding wire of a softer metallurgical bonding material, the mass of droplets consisting of about 30 to 60% of molybdenum, the remaining being bonding material, said spraying being made effected to integrally line said cylindrical surface with the mass of droplets in at least one layer, said cylindrical surface being subjected, during spraying, to a rotational motion around its geometrical axis, simultaneously with a translational motion around said geometrical axis, to produce helical displacements of spraying points throughout the entire cylindrical surface.

2. A process, as in claim 1, wherein the metallurgical bonding material is stainless steel.

3. A process, as in claim 2, wherein the mass of droplets consists of substantially equal parts of molybdenum and stainless steel.

4. A process, as in claim 3, wherein the lining of the external contact face of said piston rings presents a final finished thickness, which corresponds from 2 to 7% of the. annular width of the ring.

5. A thermal spraying lining process substantially as hereinbefore described with reference to, and as illustrated, in the accompanying drawings.